C8-C24 dicarboxylic acids (also indicated individually or collectively in this application as “long chain dicarboxylic acids”) are well known chemical intermediates widely used mainly as intermediates in the production of perfumes, lubricants, adhesives and as co-monomers for the production of various types of polymers such as for example aliphatic and aliphatic-aromatic polyesters of the diacid-diol type and polyamides of the diacid-diamine type.
At the present time these dicarboxylic acids are obtained by conventional chemical processes from various raw materials of both fossil and renewable origin. As far as processes of synthesis starting from raw materials of fossil origin are concerned, typical examples are the terminal oxidation reactions of linear hydrocarbons such as for example tridecane. As far as synthesis processes starting from raw materials of renewable energy are concerned, mention may be made by way of example of the ozonolysis of oleic acid to obtain azelaic acid and pelargonic acid, the synthesis of sebacic acid by the alkaline fusion of ricinoleic acid or metathesis reactions of monounsaturated fatty acids. The ever increasing search for new technologies having lesser environmental impact has also in recent years provided an incentive for the search for processes to obtain these long chain diacids using fermentation processes from corresponding hydrocarbons or linear monocarboxylic acids.
In all these processes, whether of the conventional chemical type or of the fermentative type, the dicarboxylic acids are generally obtained in a mixture with monocarboxylic acids which may derive from residual quantities of the starting raw material or be produced as intermediates in the synthesis reaction for the desired product.
Various methods for separating out and purifying the dicarboxylic acids so produced which provide for example for crystallisation, distillation, liquid/liquid extraction or settling have been proposed.
For example WO 01/04337 describes a process for the purification of fermentation broths for the recovery of carboxylic acids. The purification process provides for a first step in which the pH of the fermentation broth is adjusted to a value of 2 or less and a second step in which the broth at this pH is heated to a temperature of approximately 60-105° C. Under these conditions a three-phase system forms (an aqueous phase containing a few cell residues, an organic phase containing the carboxylic acids and a solid phase containing cell residues).
GB 2,016,453 instead describes a process for the purification of fermentation broths in which the broth is rendered basic, separating out a liquid phase containing the dicarboxylic acids. Subsequently the solution is acidified at pH<4, causing the dicarboxylic acids to precipitate out.
U.S. Pat. No. 6,143,532 describes a process for the purification of fermentation broths for the recovery of carboxylic acids. The purification process provides for a first step in which the pH of the fermentation broth is adjusted to a value of at least 6 and a second step in which the broth at this pH is heated to a temperature of approximately 60-75° C. Under these conditions a three-phase system forms (an upper clear aqueous phase, an intermediate organic phase containing the carboxylic acids, and a lower aqueous phase containing cell residues).
CN 102476987 discloses a process for the separation of dodecanedioic acid from a fermentation broth. The fermentation broth containing 10% of sodium salt of the dodecanedioic acid is fed to a ultrafiltration device for removing proteins and pigments. For obtaining dodecanedioic acid with sufficient purity degree the permeate containing the dicarboxylic acid is then extracted with an organic solvent, heated and acidified and then recovered by crystallization from the organic phase.
US 2011/028759 describes a process for recovery and purification of an organic acid such as lactic acid from a fermentation broth containing a salt form of the organic acid, which comprises the steps of subjecting the fermentation broth to one of ultrafiltration and microfiltration to form a first permeate, concentrating the first permeate to form a concentrated broth, subjecting the concentrated broth to a supported liquid membrane for extraction of lactic acid into a separate stream comprising an extracted solution, subjecting the extracted solution to activated carbon for colour removal, a cation exchange resin for demineralization, and an anion exchange resin for removal of anionic impurities.